1. Field
The present invention relates to an optical transmission device for use in a wavelength division multiplexed (WDM) optical transmission system.
2. Description of the Related Art
For optical transmission devices used in a WDM optical transmission system for transmitting a WDM optical signal, there are an Optical Add-Drop Multiplexer (OADM) for adding and dropping the optical signal to and from a transmission line, and an In-Line Amplifier (ILA) for amplifying and relaying the optical signal.
FIG. 10 illustrates a conventional OADM. In FIG. 10, a conventional OADM 0 includes a post-amplifier 10 that transmits an optical signal to an optical transmission line 1, a pre-amplifier 11 that receives the optical signal from the optical transmission line 1, an OADM unit 12 that realizes an OADM function, OSC units 13 that transmit and receive device control signals (such as signals for notifying the device control status or number of wavelengths), and a device controller 14 that controls various units in the device (OADM).
The OADM unit 12 includes a PD1 that monitors an input power, an optical coupler (CPL) 16 that drops the optical signal, a Wavelength Selective Switch (WSS) 17 that realizes the OADM function, an Optical Channel Monitor (OCM) 18 that monitors each channel, a PD2 that monitors an output power, a multiplexing unit (MUX) 19 that combines optical signals of different wavelengths into a WDM optical signal, and a demultiplexing unit (DEMUX) 20 that demultiplexes the WDM signal into optical signals of different wavelengths.
FIG. 6A illustrates the configuration of the WSS 17 shown in FIG. 10. In the WSS 17, a demultiplexing unit (DEMUX) 61 demultiplexes the light supplied to an input port, and a demultiplexing unit (DEMUX) 62 demultiplexes the light supplied to an Add port. Switches (SW) 63-1 to 63-n perform switching. Additionally, variable optical attenuators (VOAs) 64-1 to 64-n, which are located after the switches 63-1 to 63-n, respectively, adjust respective levels of the lights output from the switches 63-1 to 63-n, and multiplexing unit (MUX) 65 multiplexes the lights for outputting.
FIG. 11 illustrates the configuration of a conventional ILA. The same components in FIG. 11 as those in FIG. 10 are denoted by the same reference numerals, and a description of those components is omitted here.
A conventional ILA 2 is a transmission device that neither adds nor drops the optical signal. Thus, the ILA 2 has the configuration that the OADM unit 12 of the OADM 0, shown in FIG. 10, is replaced with an optical attenuator 21.
An optical amplifier used in an optical transmission system serves to amplify an optical signal which has been subjected to loss through a transmission line and components of an optical transmission device, and to compensate for the loss to keep a level of the optical signal at a desired value. A level of the output optical signal in the optical transmission system needs to be held constant to stabilize the operation of the system. As examples of a method of controlling the output signal level, there are ALC (Automatic Level Control) for holding an output level of an optical amplifier constant, and AGC (Automatic Gain Control) for holding a gain of an optical amplifier, i.e., a ratio of an output light level to an input light level, constant.
In the ALC of the optical amplifier, an output light level, i.e., a level corresponding to the total sum of levels of multiplexed optical signals, is controlled. Feedback control in the ALC is performed by applying feedback to an output so that a level per wavelength, which is obtained from the output light level and the number of wavelengths, becomes a target level.
The ALC is advantageous in having high accuracy in control of the output signal level because an output light signal level is always monitored and the feedback control is executed after converting the monitored level to a level per wavelength. However, the ALC is inferior in timeliness of control because of the necessity of executing a computation process and a feedback process of the light level. Accordingly, if the input light level undergoes a transient variation, time is required until the output light level matches with the desired light level. Additionally, a control circuit used in the ALC is more complicated than that used in the AGC, thus increasing the cost of the optical amplifier.
On the other hand, the AGC is a control for holding a ratio of the output light level to the input light level (i.e., a gain) of the optical amplifier constant. Thus, the AGC is performed so as to keep the gain constant with respect to the input signal level.
Because there is no necessity for executing computation in the control circuit, the AGC is superior in timeliness of control and causes a less influence upon the output if the input light level undergoes a transient variation. In addition, because the control circuit for the AGC is smaller, the cost of the AGC is lower than that of the ALC.
Additionally, in the AGC where the optical amplifier is controlled so as to keep the gain constant, if temperatures and properties of various components located before the optical amplifier are varied with the lapse of time and losses are changed, the control is not executed in a manner adapted for the change. Therefore, accuracy in the control of the optical signal level per wavelength deteriorates and an error occurs in the output level of the optical amplifier.
Such an error of the output light level from the desired value causes variations in the input level of the optical signal applied to the transmission line and the other components. For example, if the output level of the optical amplifier is increased, a nonlinear effect is generated in an optical fiber of the transmission line and the other components. In addition, if the output level of the optical amplifier is reduced, a signal level in another OADM falls below the required level, or an SN ratio is reduced, thus causing deterioration of the transmission performance.
The following factors are taken into consideration if the ALC or the AGC is selected as a control method for the optical amplifier used in the OADM and the ILA.
An AGC is selected as a control method for the post-amplifier in many cases because the AGC provides good transient response characteristics with respect to a variation in the number of wavelengths and is economically superior to the ALC.
However, if the AGC is selected for the post-amplifier, a variation in loss at the OADM unit, which is located before the post-amplifier, raises a problem. More specifically, since the OADM unit is made up of various optical parts to realize the add/drop/multiplex functions, loss at the OADM unit varies due to variations in losses of the parts depending on temperature and the lapse of time. Those variations change the input level of the pre-amplifier and hence deteriorate the transmission performance, as described above.
Additionally, in order to suppress the level change of the pre-amplifier, individual adjustments in the control are required by using the variable optical attenuators (VOAs) 64-1 to 64-n for individual wavelengths in the WSS 17, as shown in FIG. 6A, so that the optical signal levels of respective wavelengths are matched with one another. The VOA is inserted per channel even in an OADM node that does not require the light level control for respective wavelengths (i.e., a node in which a signal wavelength is not added and is only dropped), thus resulting in an increase of the cost.
An optical fiber for the transmission line is located before the pre-amplifier. In some cases, the optical fiber for the transmission line generates changes (such as a diurnal change and an annual change) of environment temperature with the lapse of time due to geographical conditions (environmental conditions) under which the optical fiber is laid, and the form of the laying (such as underground, undersea, or exposure to the atmosphere). Those temperature changes with a lapse of time increase and reduce the loss occurring in the optical fiber for the transmission line. In the pre-amplifier, therefore, the input light signal level is varied by a larger amount than in the post-amplifier. From the viewpoint of absorbing and compensating for such a large variation of the input signal level, it is effective to select the ALC for the pre-amplifier because of the necessity of controlling the light output level to the desired value.
FIG. 12 is a block diagram of the post-amplifier.
The post-amplifier includes an input monitor PD (photodiode) 25 that detects an input light signal level of the post-amplifier, an Er-doped fiber (EDF) 26 that amplifies an optical signal, an output monitor PD (photodiode) 27 that detects an output signal level of the post-amplifier, optical couplers 28 that branch input and output signals of the post-amplifier to the respective monitor PDs and inject an excitation light into the EDR 26, an amplifier controller 29 that detects the input and output signal levels of the post-amplifier from the input and output monitor PDs 25 and 27, calculates the difference between the detected input and output signal levels, and executes control of the excitation light and the input light applied to the EDF 26 so that the gain is controlled to match a desired gain preset in an initial state, and excitation LDs 30 that each output the excitation light for exciting the EDF 26. Additionally, the amplifier controller 29 concurrently transfers the operation state of the amplifier (including an alarm, performance, control status, control information, etc.) and information regarding the number of wavelengths to the device controller (denoted by 14 in FIGS. 10 and 11).
FIG. 13 is a block diagram of the pre-amplifier.
In FIG. 13, the same components as those in FIG. 12 are denoted by the same reference numerals.
As in the above-described post-amplifier, the pre-amplifier includes an input monitor PD (photodiode) 25 that detects an input light signal level of the pre-amplifier, an Er-doped fiber (EDF) 26 that amplifies an optical signal, an output monitor PD (photodiode) 27 that detects an output signal level of the pre-amplifier, optical couplers 28 that branch input and output signals of the pre-amplifier to the respective monitor PDs and inject an excitation light into the EDR 26, an amplifier controller 29 that detects the input and output signal levels of the pre-amplifier from the input and output monitor PDs 25 and 27, calculates the difference between the detected input and output signal levels, and executes control of the excitation light and the input light applied to the EDF 26 so that the gain is controlled to match a desired gain preset in an initial state, and excitation LDs 30 that each output the excitation light for exciting the EDF 26. Additionally, the amplifier controller 29 concurrently transfers the operation state of the amplifier (including an alarm, performance, control status, control information, etc.) and information regarding the number of wavelengths to the device controller (denoted by 14 in FIGS. 10 and 11).
Regarding an optical transmission system including optical amplifiers in previous and subsequent stages, for example, Japanese Unexamined Patent Application Publication No. 2000-22639 discloses a technique of detecting a signal loss by a dispersion compensator disposed between the optical amplifiers and compensating for the detected loss. In addition, Japanese Unexamined Patent Application Publication No. 8-248455 discloses a two-stage optical amplifier in which light power dependency upon wavelength is removed.
A significant factor causing deterioration of the characteristics in the OADM resides in that the output of each amplifier is varied and the input level applied to the transmission line is changed from the design value. While the ALC is effective in controlling the output variation as described above, it is effective to employ the AGC in the post-amplifier in consideration of, e.g., a variation in a transient response. For that reason, generally, the post-amplifier is controlled on the basis of AGC (AGC-controlled) and the pre-amplifier is controlled on the basis of ALC (ALC-controlled).
A level adjustment is required in the case of AGC-controlling the post-amplifier and ALC-controlling the pre-amplifier for matching the respective signal levels with one another after demultiplexing into individual channels in the WSS of the OADM unit, in order to remove the loss variation of the OADM unit with the lapse of time.
If the level adjustment is executed after the demultiplexing into individual channels in the WSS of the OADM unit, the VOA function is required after each of all the switches in the WSS, as shown in FIG. 6A. This raises the problem of cost efficiency in, particularly, the OADM that does not require the Add function.
Additionally, in the ILA, an input to the post-amplifier may deviate from the desired value and a fiber input level may be varied depending on the temperature, the lapse of time, and an individual variation of the optical attenuator disposed between the pre-amplifier and the post-amplifier. Such a variation of the fiber input level causes significant deterioration of the transmission performance.